Poppet valves are used in a variety of hydraulic systems such as those used to control different systems on work machines. A poppet valve typically consists of a housing with at least one input and one output hydraulic port. Inside the housing is a poppet valve member seated in a valve seat such that when the poppet valve member is in contact with the valve seat, the input and the output ports are not fluidly connected. When the poppet valve member is moved away from the valve seat by an actuator, then the input and output ports are fluidly connected and hydraulic fluid can flow across the valve seat. Typically, the housing also contains a control chamber hydraulically connected in a number of different manners, such as a position follower model described in U.S. Pat. No. 6,745,992 B2, a flow amplifying model described in U.S. Pat. No. 5,819,532, or a force feedback model as in U.S. Pat. No. 6,869,060 B2, all of which involve the poppet valve member being exposed to hydraulic pressure on at least one control hydraulic surface. In this manner the motion of the poppet valve member can be at least partially controlled and de-sensitized to differences in pressure between the input and the output ports.
A problem with these methods of controlling the poppet valve member is that the pump and line pressure changes can affect poppet control volume dynamics. This occurs because the control volume is always fluidly connected to the hydraulic system. As the pressure in the system fluctuates, the poppet valve member may move at differing rates due to the hydraulic connections of the ports to the control chamber, making accurate control difficult and unpredictable. This same problem renders it difficult to maintain the poppet valve member at a selected location away from its seat.
One possible solution to this problem is to use spool valves rather than poppet valves in hydraulic systems, such as that described in U.S. Pat. No. 5,186,212. Spool valves include a spool valve member that slides back and forth inside a bore of a housing to open and close fluid ports. An advantage of spool valves is that they are pressure balanced and can therefore be precisely positioned regardless of pressure differences. Spool valves, however, have a disadvantage in that they necessarily have a radial clearance between the spool valve member and the housing, so they inherently leak. This can cause problems when the spool valves are used in work machine applications such as loaders, such as where it might be desirable to keep the loader bucket in a lifted position over a prolonged period of time.
The present disclosure is directed to one or more of the problems set forth above.